Toner utilized in development in the electrographic process is generally prepared by mixing and dispersing a colorant and a charge enhancing additive into a thermoplastic binder resin, followed by micropulverization. As the thermoplastic binder resin, several polymers are known including polystyrenes, styrene-acrylic resins, styrene-methacrylic resins, styrene-butadiene resins, polyesters, epoxy resins, acrylics, urethanes and copolymers thereof. As the colorant, carbon black is normally utilized and as the charge enhancing additive, alkyl pyridinium halides, distearyl dimethyl ammonium methyl sulfate, and the like are known.
To fix the toner to a support medium, such as a sheet of paper or transparency, hot roll fixing is commonly used. In this method, the support medium carrying a toner image is transported between a heated fuser roll and a pressure roll with the image face contacting the fuser roll. Upon contact with the heated fuser roll, the toner melts and adheres to the support medium forming a fixed image. Such a fixing system can be very advantageous in heat transfer efficiency.
Fixing performance of the toner can be characterized as a function of temperature. The lowest temperature at which the toner adheres to the support medium is referred to as the Cold Offset Temperature (COT), and the maximum temperature at which the toner does not adhere to the fuser roll is referred to as the Hot Offset Temperature (HOT). When the fuser temperature exceeds HOT, some of the molten toner adheres to the fuser roll during fixing and is transferred to subsequent substrates containing developed images resulting, for example, in blurred images. This undesirable phenomenon is referred to as offsetting. Between the COT and HOT of the toner is the Minimum Fix Temperature (MFT) which is the minimum temperature at which acceptable adhesion of the toner to the support medium occurs, as determined by, for example, a creasing test. The difference between MFT and HOT is referred to as the Fusing Latitude.
The hot roll fixing system and a number of toners used therein can, however, exhibit several problems, for example the binder resins in the toners can require a relatively high temperature in order to be affixed to the support medium. This may result in high power consumption, low fixing speeds, and reduced life of the fuser roll and fuser roll bearings. Also, offsetting can be a problem, and further, toners containing vinyl type binder resins, such as styrene-acrylic resins, may have an additional problem which is known as vinyl offset. Vinyl offset occurs when a sheet of paper or transparency with a fixed toner image comes in contact for a period of time with a polyvinyl chloride (PVC) surface containing a plasticizer used in making the vinyl material flexible such as, for example, in vinyl binder covers, and the fixed image adheres to the PVC surface.
There is a need for a toner with low fix temperature and high offset temperature (or wide fusing latitude) and superior vinyl offset property, and processes for the preparation thereof. Toners which function at lower temperatures would reduce the power needed for operation and increase the life of the fuser roll and the high temperature fuser roll bearings. Additionally, such low melt toners, that is, for example, toners with a MFT lower than 200.degree. C., and preferably lower than 160.degree. C. would reduce the volatilization of release oil such as silicone oil which may occur during high temperature heating operation and which can cause problems when the volatilized oil condenses in other areas of the machine. In particular, toners with a wide fusing latitude and with acceptable toner particle elasticity are needed. Toners with wide fusing latitude can provide flexibility in the amount of oil needed as release agent and can minimize copy quality deterioration related to toner offsetting to the fuser roll. These toners are provided with the present invention and processes thereof.
To prepare lower fix temperature resins for toner, the molecular weight of the resin may be lowered. Low molecular weight and amorphous polyester resins and epoxy resins have been used to prepare low temperature fixing toners. For example, attempts to generate toners utilizing polyester resins as binder are disclosed in U.S. Pat. No. 3,590,000 to Palermiti et al. and U.S. Pat. No. 3,681,106 to Burns et al. The minimum fixing temperature of polyester binder resins can be rendered lower than that of other materials, such as styrene-acrylic resins. However, this may lead to a lowering of the hot offset temperature and, as a result, decreased offset resistance. In addition, the glass transition temperature of the resin may be decreased, which may cause the undesirable phenomenon of blocking of the toner during storage.
To prevent fuser roll offsetting and to increase fusing latitude of toners, modification of the binder resin structure by conventional polymerization processes, for example by branching, crosslinking, and the like, has been attempted. For example, in U.S. Pat. No. 3,681,106 to Burns et al., a process is disclosed whereby a polyester resin was improved with respect to offset resistance by nonlinearly modifying the polymer backbone by mixing a trivalent or more polyol or polyacid with the monomer to generate branching during polycondensation. However, an increase in degree of branching may result in an elevation of the minimum fix temperature. Thus, any initial advantage of low temperature fix may be diminished.
Another method of improving offset resistance is by crosslinking during polymerization. In U.S. Pat. No. 3,941,898 to Sadamatsu et al., for example, a crosslinked vinyl type polymer prepared using conventional crosslinking was used as the binder resin. Similar disclosures for vinyl type resins are presented in U.S. Pat. Nos. 31,072 (a reissue of 3,938,992); 4,556,624; 4,604,338 and 4,824,750. Also, disclosures have been made of crosslinked polyester binder resins using conventional polycondensation processes for improving offset resistance, such as for example in U.S. Pat. No. 3,681,106.
While significant improvements can be obtained in offset resistance and entanglement resistance, a major drawback may ensue with these kinds of crosslinked resins prepared by conventional polymerization, both vinyl type processes including solution, bulk, suspension and emulsion polymerizations and polycondensation processes. In these processes, monomer and crosslinking agent are added to the reactor. The crosslinking reaction is not very fast and chains can grow in more than two directions at the crosslinking point by the addition of monomers. Three types of polymer configurations are produced--a linear and soluble portion called the linear portion, a crosslinked portion which is low in crosslinking density and, therefore, is soluble in some solvents like tetrahydrofuran, toluene and the like, and is referred to as a sol, and a portion comprising highly crosslinked gel particles, which is not soluble in substantially any solvent like tetrahydrofuran, toluene and the like, and is referred to as the gel. The second portion with low crosslinking density (sol) is responsible for widening the molecular weight distribution of the soluble part which results in an elevation of the minimum fixing temperature of the toner. Also, a drawback of these processes, which are not accomplished under high shear, is that as more crosslinking agent is used the gel particles or very highly crosslinked insoluble polymer with high molecular weight increase in size. The large gels can be more difficult to disperse pigment in, causing unpigmented toner particles during pulverization, and toner developability may thus be hindered. Also, in the case of vinyl polymers, the toners produced often show vinyl offset.
Crosslinked polyester binder resins prepared by conventional polycondensation reactions have been provided for improving offset resistance such as, for example, as illustrated in U.S. Pat. No. 3,681,106. As with crosslinked vinyl resins, increased crosslinking as obtained in such conventional polycondensation reactions may cause the minimum fix temperature to increase. When crosslinking is accomplished during polycondensation using tri-or polyfunctional monomers as crosslinking agents with the polycondensation monomers, the net effect is that apart from obtaining highly crosslinked high molecular weight gel particles, which are not soluble in substantially any solvent, the molecular weight distribution of the soluble part widens due to the formation of sol or crosslinked polymer with a very low degree of crosslinking, which is soluble in some solvents. These intermediate molecular weight species may result in an increase in the melt viscosity of the resin at low and high temperature, which can cause the minimum fix temperature to increase. Furthermore, gel particles formed in the polycondensation reaction which is carried out using conventional polycondensation in a reactor with low shear mixing can grow rapidly with increase in degree of crosslinking. As in the case of crosslinked vinyl polymers using conventional polymerization reactions, these large gel particles may be more difficult to disperse pigment in, resulting in unpigmented toner particles after pulverization, and thus hindering developability.
Crosslinked polyester binder resins prepared by a reactive melt mixing process has been disclosed in U.S. Pat. No. 5,227,460, the disclosure of which is totally incorporated herein by reference. In this process in embodiments, the crosslinking reaction is accomplished with a chemical initiator when the polymer is in the molten state. The crosslinked resin comprises crosslinked portions and linear portions. The crosslinked portions comprise very high molecular weight densely crosslinked gel particles having an average diameter of less than about 0.1 micron and are insoluble in substantially any solvent. The linear portion comprises low molecular weight resin soluble in various common solvents. Substantially no portion of the resin comprises sol or polymer with low degree of crosslinking. The crosslinked portions or microgel particles are prepared in such a manner that there is substantially no distance between the polymer chains. This crosslink structure is different from conventional crosslinking in which the crosslink distance between chains is quite large with several monomer units. The highly crosslinked microgel particles distributed throughout the linear portion impart elasticity to the resin which improves the resin offset properties, while not substantially effecting the resin minimum fix temperature. This melt mixing process in U.S. Pat. No. 5,227,460 is in embodiments a reactive melt mixing process whereby a base resin is converted into a resin with crosslinked fraction.
Many processes are known for effecting polymerization reactions, including reactive melt mixing processes, for both initial polymerization reactions employing monomers or prepolymers, and for polymer modification reactions, such as grafting, coupling, crosslinking and degradation reactions. The process is known as reactive extrusion process when the melt mixing device is an extruder. The reactive extrusion process is particularly advantageous for polymer modifications in many respects. The modification generally takes place when the polymer is in molten state, thus eliminating the use of a large amount of solvent whose handling is both difficult and costly. The extrusion process is inherently easier to control as compared to a large polymerization reactor vessel.
Melt mixing processes for preparing toner based on crosslinked polyester binder resin prepared by conventional approach in a reactor or by reactive processing are known, such as those disclosed and illustrated in U.S. Pat. No. 5,376,494 and U.S. Pat. No. 5,227,460, the disclosures of which are totally incorporated herein by reference. In these processes, the partially crosslinked resin prepared by the reactive extrusion process or any conventional methods is subsequently melt blended again with a colorant, charge enhancing additives and the like to result in a toner mixture before pulverizing operation to obtain toner particles. However, even with crosslinking via reactive extrusion where very small gel domains are produced, large aggregates of gel domains, sometimes on the order of 1 to 5 microns, can form in subsequent melt mixing processes and under certain extrusion conditions.
These large gel aggregates are difficult for pigment particles and other toner additives to penetrate, thus generating unpigmented particles during pulverization. The lack of pigment can result in differences in specific gravity and triboelectric charging between the unpigmented particles and toner particles containing pigment and other additives. As a result, some of the unpigmented gel particles will be removed with fines during classification operation affecting, therefore, the toner fusing property. Furthermore, the unpigmented particles that remain in toner will charge differently from toner particles containing pigment and additives. Thus, the development properties of the toner will also be adversely affected.
Therefore, there remains a need for a fabrication processes which can eliminate the occurrence of unpigmented gel particles and avoid or minimize the other disadvantages illustrated herein.